In general, rotorcraft lift rotors comprise a hub driven in rotation about a drive axis by an outlet shaft of a main gearbox, referred to as a drive shaft, and at least two blades that are fastened to the hub via suitable hinges. For example, each blade has a sleeve that is hinged to the hub by means of a laminated elastomer thrust bearing.
It is recalled that assuming that each blade is embedded in a hub, the resulting rotor is a rigid rotor. While hovering, the distribution of aerodynamic forces along a blade gives rise to bending moments in flapping and in drag that present very large values at the root of the blade because of the way circumferential speed increases in proportion to the radius of the rotor.
Furthermore, in level flight, the so-called “advancing” blade exerts lift that is greater than the lift of the so-called “retreating” blade because of the difference in air speeds, as described below.
Consequently, the resultant of the aerodynamic forces exerted on a blade does not have the same value at all azimuth positions, nor does it have the same point of application: the bending moment at the root of the blade is thus high and variable, thereby generating alternating mechanical stresses that give rise to a fatigue phenomenon that is harmful to materials. Furthermore, the resultant of the aerodynamic forces on all of the blades is no longer carried by the drive axis of the rotor, thereby creating a roll moment, which increases with increasing speed of advance of the rotorcraft and which can make it difficult to balance the rotorcraft in level flight.
In order to remedy those drawbacks, it is known to hinge each blade to the hub about an axis that is perpendicular to the drive shaft and that is referred to as the vertical flapping axis, which axis corresponds to a vertical flapping hinge capable of transferring lift but under no circumstances of transferring a bending moment. Consequently, if a blade is hinged to the hub for flapping, the flapping bending moment is zero where it is attached, i.e. at the flapping hinge. To ensure a blade remains in equilibrium, centrifugal forces maintain the blade after it has flapped up a certain amount so that the resultant of the lift and of the centrifugal forces is oriented along said flapping axis, thereby causing conicity a0 to appear.
Under such conditions, there is no longer any large roll moment in level flight and the blades no longer rotate in a plane, but rather their outer ends describe a cone that is very flat.
During hovering, the conicity of the lift rotor is unvarying around one revolution, which means that the center of gravity of each blade as seen from above describes a circle on each revolution.
However, in order to perform level flight, the cone described by the blades of the lift rotor needs to be tilted by causing the pitch of the blades to vary cyclically. This requires a pitch hinge having its axis substantially parallel to the span of the corresponding blade. This new degree of freedom enables the lift of a blade to be controlled by acting on a general pitch control and/or by causing the pitch to vary cyclically, thus enabling the plane of rotation of the blades to be controlled so as to describe a cone of geometrical axis that no longer coincides with the drive axis as represented by the rotor shaft.
In this context, documents GB 1 188 947 and GB 1 188 946 describe means for twisting a blade by longitudinally moving an element that is suitable for turning ribs of the blade, said twisting nevertheless not being cyclic. In contrast, document WO 02/12063 describes a device for cyclically varying the pitch of a blade.
As mentioned above, the geometrical axis of the cone described by the blades may depart from the drive axis. Under such conditions, and unlike when hovering, the end of each blade is at a distance from the rotor shaft that varies. The projection of the end of each blade onto a plane that is perpendicular to the drive axis then no longer describes a circle, but rather an ellipse on each revolution. Thus, the projection of the end of each blade needs to describe arcs of different lengths over equal periods of time, thereby generating large alternating inertial bending moments on the blades in their plane. In order to avoid such moments, that give rise to undesirable mechanical stresses, it is necessary to hinge each blade in drag. Such a drag hinge operates about a drag axis that is substantially parallel to the rotor axis and that is consequently substantially perpendicular to the drag forces. To enable such a blade to be driven from the drive shaft, it is naturally necessary for the drag hinge not to coincide with the rotor axis, which means that the drag axis needs to be offset, or to present eccentricity e. It should be observed that this hinge enables a blade to move angularly in drag about the drag hinge under the effect of inertial forces and aerodynamic forces.
It should be observed that the angular moment in drag of a blade about the drag hinge under the effect of inertial forces and aerodynamic forces constitutes a source of vibration and of noise.
Devices that are bulky, heavy, and expensive, based on dampers and referred to as “drag adapters” are thus used to reduce such vibration and noise.
The technological background includes the document JP 4 215 596 and the document by B. Popescu et al., “Several considerations regarding the variable blade length rotor”, Journal of Aircraft, AIAA, Reston, Va., USA, Vol. 31, No. 4, Jul. 1, 1994, pp. 975-977.